1. Field of the Invention
The present invention relates to an optical apparatus and an image capturing apparatus having a function of correcting a blur of a captured image due to camera shake or the like.
2. Description of the Related Art
Optical image stabilization, which changes the position of an optical axis in accordance with camera shake by moving a correction lens, is known as a method of correcting an image blur due to camera shake or the like.
In optical image stabilization, the ratio of the displacement amount of an image to that of the correction lens is referred to as decenter sensitivity, and represented by a function of the focal length and the imaging magnification. That is, the displacement amount dL of the correction lens can be represented by sensitivity k, which is represented by the function of the focal length f and the imaging magnification β, and a shake angle θ as:dL=k(f,β)×θ  (1)where the focal length f and the imaging magnification β are values determined by the positions of a zoom lens and a focus lens.
In other words, the displacement amount of the correction lens varies depending on the positions of the zoom lens and the focus lens even when correcting an image blur due to the shake angle of the same camera. Hence, in order to implement satisfactory image stabilization, it is necessary to drive and control the correction lens in accordance with the sensitivity determined by the focal length and the object distance.
Japanese Patent Laid-Open No. 6-308564 proposes a method presented below as a correction method of a correction lens in accordance with the sensitivity. That is, image stabilization is performed by dividing the positions of a zoom lens and a focus lens into a finite number of regions, storing, as a reference table, the sensitivity corresponding to each region in a memory, and using the value of that sensitivity.
In recent years, along with development of a technique of correcting shake of an image capturing apparatus, an image stabilization function of correcting not only a blur of a captured image caused by camera shake when a photographer is in a still state but also a large image blur which is generated in shooting while the photographer is walking has become widespread.
In a technique disclosed in Japanese Patent Laid-Open No. 6-308564 mentioned above, image stabilization is performed by calculating an object distance from the focus lens position to define the sensitivity of the correction lens, and driving the correction lens based on equation (1). However, on the wide side of a lens system of an inner focus type which is becoming widespread, a change in the object distance with respect to the focus lens position is large and the calculation precision of the object distance is low. In addition, during walking shooting, a focus variation for adjusting the focus occurs frequently because a change in an object is greater than in still shooting. Consequently, the following problem arises in the above conventional method.
That is, when walking shooting on a wide side viewing angle, there is a phenomenon in which the object distance undergoes a greater and more frequent change than in still shooting, and sensitivity also undergoes a frequent change accordingly. This is because besides a variation in the focus lens is frequent, the precision of the object distance is low. The phenomenon will be described in detail with reference to examples of an optical characteristic shown in FIGS. 7 and 8.
FIG. 7 is a graph showing the focus lens position to attain focusing at an arbitrary object distance when the focal length, that is, the zoom lens position is changed in a lens system of the inner focus type. The abscissa represents the zoom lens position, and the ordinate represents the focus lens position. Of the curves shown in FIG. 7, the one at the bottom corresponds to a position to attain focusing at an infinite object distance, and they correspond to a position for focusing at an object distance nearer to an image sensing plane as they go to the top. As is apparent from FIG. 7, focus lens positions, which attain focusing at an infinity object distance from approximately 80 cm to ∞ are more dense on the wide side than on the tele side. This means that the depth of field is large on the wide side, and indicates that an out-of-focus state hardly occurs even when the focus lens is subject to shaking to some extent. However, this also means that the object distance detected from the focus lens position easily changes by moving the focus lens just a little, and its precision is low.
FIG. 8 is a graph showing a state in which sensitivity to the object distance changes with respect to a change in the focal length. The abscissa represents the zoom lens position, and the ordinate represents the sensitivity. The change in the sensitivity varies depending on optical conditions, such as a lens configuration. For example, an example shown in FIG. 8 indicates that the sensitivity differs depending on the object distance on the wide side. Under the condition of such optical characteristic, when walking shooting on the wide side viewing angle, a wobbling operation of the focus lens frequently occurs due to a change in the object in a frame. Moreover, since the precision of the object distance is low because of the characteristic in FIG. 7, object distance information detected based on the focus lens position also changes frequently, and along with that, the sensitivity also changes frequently because of the characteristic in FIG. 8.
In practice, the object distance does not change suddenly and frequently, and hence sensitivity for correcting an image blur should be almost fixed. However, because of the optical characteristics above, the change in sensitivity also occurs as the object distance changes. Therefore, a moving amount (a correction amount) of the correction lens calculated based on equation (1) varies even when the shake angle is constant, and as a consequence the correction amount of an image plane on an image sensor also changes and a satisfactory image stabilization effect cannot be obtained.